[Python-Dev] Counting collisions for the win (original) (raw)

Frank Sievertsen pydev at sievertsen.de
Mon Jan 23 19:58:25 CET 2012

On 23.01.2012 19:25, Glenn Linderman wrote:

So this sounds like SafeDict, but putting it under the covers and automatically converting from dict to SafeDict after a collision threshold has been reached. Let's call it fallback-dict.

and costs: * converting the dict from one hash to the other by rehashing all the keys.

That's not exactly what it does, it calls the randomized hash-function only for those keys, that that didn't find a free slot after 20 collision. And it uses this value only for the further collision resolution.

So the value of hash() is used for the first 20 slots, randomized_hash() is used after that.

1st try: slot[i = perturb = hash]; 2nd try: slot[i=i * 5 + 1 + (perturb >>= 5)] 3rd try: slot[i=i * 5 + 1 + (perturb >>= 5)] .... 20th try: slot[i= i * 5 + 1 + (perturb >>= 5)] 21th try: slot[i= perturb = randomized_hash(key)] <---- HERE 22th try: slot[i= i * 5 + 1 + (perturb >>= 5)]

This is also why there is no conversion needed. It's a per-key/per-lookup rule.

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